Signature schemes are arguably the most crucial cryptographic primitive, and devising tight security proofs for signature schemes is an important endeavour, as it immediately impacts the feasibility of deployment in real world applications. Hash-then-sign signature schemes in the Random Oracle Model, such as RSA-FDH, and Rabin-Williams variants are among the fastest schemes to date, but that unfortunately do not enjoy tight security proofs based on the one-wayness of their trapdoor function; instead, all known tight proofs rely on variants of the (non-standard) Φ-Hiding assumption.
As our main contribution, we introduce a family of hash-then-sign signature schemes, inspired by a lossy trapdoor function from Freeman et al. (JoC’ 13), that is tightly secure under the Quadratic Residuosity assumption. Our first scheme has the property of having unique signatures, while the second scheme is deterministic with an extremely fast signature verification, requiring at most 3 modular multiplications.